The COSMOS and APM plate measuring machines were both flying-spot systems, in which a beam of light (either incoherent from a CRT as on COSMOS or a laser spot as on APM) is rapidly scanned over a small area of a plate, and the amount of transmitted light is measured. It is extremely difficult to meet the above performance criteria with a flying spot system, for two reasons. First, the precise location of the flying spot is not precisely known as a function of time, resulting in small-scale positional errors. Second, because the light-measurement system is diffuse it is susceptible to scattered light, and especially any halo of scattered light around the spot. This problem was particularly acute on COSMOS.
A direct imaging system removes both these problems. The imaging system can be made rigid, so that small-scale random position errors are eliminated (although systematic errors may remain) and because an image is formed of the emulsion, any light which is transmitted through the emulsion contributes to forming a correct image of the plate, regardless of the origin of that light. Only scattered light introduced into the optical path between the emulsion and the detector can degrade the image quality, and we shall see below that with the correct choice of imaging system such light can be eliminated very effectively.